1. Field of the Invention
The present invention relates to an exhaust-emission purification apparatus of an engine (internal combustion engine) mounted in an automobile or the like.
2. Description of Related Art
An exhaust-emission purification apparatus employing a carrier carrying a noble metal (e.g., platinum, rhodium, and so on) or the like is known as a purification apparatus to purify exhaust gas of an automotive engine. To purify noxious exhaust-gas components utilizing such a noble-metal catalyst, it is generally considered to be necessary that the temperature of the catalyst rise to 350.degree. C. or more, which is an activation temperature.
Immediately after engine starting, however, the catalyst has not reached the activation temperature, and so a problem exists whereby noxious exhaust-gas components of hydrocarbons (hereinafter abbreviated as "HC") and the like remain essentially unpurified and are emitted from the automobile.
In this regard, Japanese Patent Laid-Open Publication No. Hei 4-17710 provides a catalytic apparatus in an emissions system of an engine which includes a bypass passage in a downstream side thereof, and disposes in this bypass passage an adsorption device having an adsorbent to adsorb noxious exhaust-gas components which cannot be purified by the catalyst device when cold-staring the engine.
According to this device, a switching valve is disposed on a downstream side of the adsorption device for noxious exhaust-gas components, and along with this, the passage between this adsorption device and the switching valve is communicated to an intake side of an engine by an exhaust reflux passage, and moreover HC adsorbed by the adsorption device is separated at a downstream side thereof by the switching valve by opening the exhaust reflux passage, so that the separated HC is refluxed through the exhaust reflux passage to the intake side of the engine.
Herein, adsorption conditions of favorable efficiency are generally obtained when adsorption-component concentration in ambient gas is high and temperature is low, whereas rapid separation conditions are obtained when the absorption (separation) component concentration in ambient gas is low and temperature is high.
However, according to the foregoing Japanese Patent Laid-Open Publication No. Hei 4-17710, because the above-described structure causes noxious exhaust-gas components such as HC to separate at the downstream side of the adsorption device irrespective of whether a large amount of these noxious exhaust-gas components such as HC have been adsorbed in the upstream side of the adsorption device, a significant amount of time is required for the separation of noxious components.
Furthermore, because the adsorption device is disposed in the bypass passage of the main exhaust gas passage, assuming a structure wherein it is difficult for heat of exhaust gas flowing through the main exhaust gas passage to be transmitted to the adsorption device during the separation of noxious components, the temperature of the adsorbent does not rise during separation, and still more time is required for the separation of noxious components.
Additionally, a switching valve is provided in the downstream side of the adsorption device, and so the possibility also exists that when particulates and the like in the exhaust gas adhere to the switching valve and the sealing performance of the switching valve during opening and closing deteriorates, noxious components which have separated from the adsorption device will pass unaltered through the main exhaust passage and be released into the atmosphere.
Moreover, according to Japanese Patent Laid-Open Publication No. Hei 5-44446, the direction of gas flow in the interior of the adsorbent is reversed during adsorption of HC and the like and during separation of HC and the like, and separation of HC and the like from the adsorbent is performed. However, because the atmosphere is utilized as the gas used in separation, there exists the problem whereby the adsorbent is cooled, rapid separation conditions of HC are not fulfilled, and much time is required for separation.
Furthermore, according to Japanese Utility Model Laid-Open Publication No. Hei 3-59418 as well, the direction of gas flow is reversed during HC adsorption and during separation, but because exhaust gas purified by the catalyst is utilized as the gas used in separation, the temperature of the adsorbent becomes high and separation time can also be shortened.
However, because this apparatus has a complex exhaust-passage structure on an upstream-side portion of a catalyst device which constitutes a main mode of operation of exhaust purification, the amount of exhaust heat needed to cause the temperature of the catalyst device to rise is correspondingly insufficient, the temperature rise of the catalyst device which constitutes the main mode of operation is delayed, and activation of the catalyst is not performed rapidly.